Atherosclerosis, sometimes called the hardening or clogging of the arteries, is an accumulation of cholesterol and fatty deposits, called plaque, on the inner walls of the arteries. Atherosclerosis causes a partial or total blockage of the arteries and, consequently, a reduced blood flow to the heart, legs, kidneys, or brain.
Traditionally, clogged arteries has been treated with surgical procedures that involve the removal of the diseased arterial tract. Angioplastic procedures, during which a stent is inserted in the diseased portion of the artery, have gained increased acceptance during the last two decades because of the reduced complexity of the procedure, with a consequent reduction in risk and discomfort to the patient.
Referring first to FIG. 1, a stent 10 is a small tubular element that typically has a cylindrical structure 12 and that, once placed within a blocked vessel, acts as a scaffold to keep the vessel open. Stent 10 may be implanted in a bodily vessel by disposing the stent over a balloon tipped catheter and by subsequently inflating the balloon at a target location. Alternatively, stent 10 may be caused to self-expand without the use of a balloon by manufacturing stent 10 from a shape memory material and disposing stent 10 over a catheter in a contracted delivery configuration, successively allowing stent 10 to self-expand at the target location.
One type of self-expanding stent is produced from a superelastic material and is compressed inside a sheath into the contracted delivery configuration. When the stent is released from the sheath, the flexible material causes the stent to spring back to its original shape and size before compression. Another type of self-expanding stent is produced from a thermo-elastic shape-memory material that is formed into a desired size and shape and is then annealed at a temperature higher than a transition temperature. After cooling the stent to a temperature below the transition temperature, the stent becomes soft and can be reduced to a smaller size by compression, so that it can be delivered to the target location, where the stent is warmed to a temperature above the transition temperature, returning to the preprogrammed size and shape.
The following description relates to a balloon expandable stent, but it will be appreciated that a self-expanding stent may be employed instead of the balloon expandable stent, when a clinician believes that self-expansion provides more desirable properties than balloon expansion, for example, due to stent recoil problems.
With reference to FIGS. 2A-2C, the placement of a stent 22 into an artery 14 having a buildup of plaque 16 is performed with a very thin tube called catheter 18. The distal end of catheter 18 contains a deflated balloon 20, onto which stent 22 is disposed in a contracted delivery configuration. Balloon tipped catheter 18 is inserted, using local anesthesia, into artery 14 with a needle puncture, typically in the groin, and is guided through the vascular system until its tip reaches the blocked tract of the artery. Balloon 20 is then inflated, deploying stent 22 until contact with the walls of artery 14 is achieved. The deployment of stent 22 causes plaque 16 to become compressed against the walls of artery 14 and also causes artery 14 to be widened and supported, improving blood flow 24.
Carotid arteries may experience a significant amount of plaque build-up. The nature of the plaque varies considerably, but in a significant number of cases plaque may not only narrow the carotid arteries, but pieces of the plaque may break away and cause neurological impairment by blocking blood flow to specific areas of the brain. With reference to FIGS. 3A-3C, stenting of carotid artery 26 is complicated by the bifurcated disposition of carotid artery 26. As shown more particularly in FIG. 3A, carotid artery 26 travels from the heart to the head through the neck, supplying vital oxygen and glucose-rich blood to the parts of the brain where thinking, personality, speech, and sensory and motor functions reside. As shown in greater detail in FIGS. 3B-3C, plaque on carotid arteries 26 can be removed through a surgical procedure called carotid endarterectomy, which involves cutting an incision in the neck of the patient and in artery 26 at the site of the carotid artery blockage. Artery 26 is isolated, and plaque 28 and diseased portions of artery 26 are surgically removed. Artery 26 is then sewn back together, improving blood flow to the brain and lessening the probability of a stroke.
An alternative procedure for treating blockage of the carotid arteries is carotid angioplasty, during which a carotid artery stent is inserted inside carotid artery 26 at the site of blockage, providing a scaffolding of carotid artery 26 that keeps carotid artery 26 open and that reduces the risk of plaque becoming loose and reaching brain vessels. During carotid angioplasty, only local anesthesia is used as a tiny incision is made in the groin, and the patient remains alert and awake during the procedure, reducing recovery time.
The bifurcated anatomy and frequent movements of carotid artery 26 require that stents with a high degree of flexibility be employed for carotid angioplasty. Carotid stents in the prior art are formed as a metal mesh or, in general, as a web structure that provides some degree of flexibility. Each of the stent designs in the prior art, however, increases flexibility by increasing cells size in the mesh or web structure. Therefore, whenever stent flexibility is increased in stent in the prior art, scaffolding support is affected negatively.
An example of prior art stent is disclosed in U.S. Pat. No. 5,104,404 to Wolff, which teaches an articulated stent in which stent segments, formed by diamond-shaped cells disposed in ring form, are connected one to the other at some but not all of the tips of the diamond-shaped cells. This arrangement provides for a stent with a high degree of longitudinal flexibility, but also for limited support to the arterial walls at the junctions areas between the different stent segments.
U.S. Pat. No. 5,449,373 to Pinchasik also discloses a stent formed by a plurality of longitudinal stent segments that are connected one to the other by arched segments. This arrangement enables only a limited degree of longitudinal translation between the stent segments, reducing stent flexibility, and also causes one stent segment to rotate in relation to the neighboring segment when tensile or compressive forces are applied to the stent.
U.S. Pat. No. 6,190,403 to Fischell discloses a stent having a plurality of stent segments that are disposed in longitudinal order. Each of the stent segments is formed by longitudinally-oriented cells disposed circumferentially and is joined to a neighboring stent segment by sinusoidal connectors connecting cell tips that are longitudinally aligned one with the other. The stent of the '403 patent provides an elevated degree of scaffolding to the arterial walls, but its structure allows only a limited degree of longitudinal flexibility, due to the limited extent of longitudinal translation between the stent segments that is possible when a compressive force is applied, because this translation is limited to the distance between adjacent stent segments.
U.S. Pat. No. 7,029,493 to Majercak et al. discloses a stent structure having a plurality of wavy rings, joined one to the other by connecting neighboring crests of adjacent rings with curvilinear connectors. Because the ring crests are longitudinally aligned, the degree of motion of the different rings under compressive stress is necessarily limited to the distance between neighboring crests.
Therefore, it would be desirable to provide a stent that generates an elevated degree of scaffolding to a bodily vessel while remaining highly flexible.
It would also be desirable to provide a stent, in which adjacent stent segments can translate one with respect to the other for a distance greater than the distance existing between neighboring rings when the stent is at rest.